Why did Billy swoon and collapse?

Are you referring to chapter 18? Now the rescue party notices that a Mr. Benson is not with them, so they start whooping for him and he answers. Soon he joins them, breathless as if he has been running. He immediately starts jabbering about two white figures he has seen that he thought were ghosts. He calms down and explains that he saw two hounds, frozen solid. Billy runs to his father at this news and faints. When he comes to the man adds that the dogs are alive, and he apologizes for the frightful way he reported his experience.